A saddle field source of the kind referred to above is suitable for generating an ion beam as well as a neutral particle beam by means of an internal charge exchange. Such a saddle field source is disclosed in the article of J. Franks entitled "Atom beam source" published in the journal "Vacuum", Volume 34, Numbers 1-2, (1984), pages 259 to 261. The above-mentioned saddle field source as strictly an ion source is also disclosed in U.S. Pat. Nos. 3,944,873 and 4,354,113 and in the article of W. Ensinger entitled "Ion sources for ion beam assisted thin-film deposition" published in "Review of Scientific Instruments", Volume 63, Number 11, (November 1992), pages 5217 to 5233 (FIGS. 16 and 17).
Conventionally, saddle field sources have a high symmetry and especially a mirror plane which contains the anode arrangement and a mirror plane perpendicular thereto which contains the exit opening. The chamber defining the cathode and the anode arrangement are substantially mirror-symmetrical to both planes. In this way, the electric field defines a saddle point in the chamber within the anode arrangement. The symmetry produces the condition that the ion particle current or neutral particle current exiting via the exit opening corresponds to a current of the same magnitude on the opposite-lying wall surface.
This return current is lost and possibly produces damage to the wail of the chamber so that the efficiency of the ion or particle source is reduced. U.S. Pat. No. 3,944,873 suggests in FIG. 6 to utilize a hole in the rear wall for monitoring purposes.
The cited patents disclose arrangements which increase the symmetry of the saddle field sources, especially with respect to the electric field, by means of additional shielding electrodes.
Saddle field sources are described having spherical or cylindrical chambers with axial exit openings for conically-shaped ion or particle beams. Saddle field sources having cylindrical chambers and an exit slit on the circular cylindrical wall for a wide curtain-like ion beam are also described.